spi: Move DM_SPI_FLASH to Kconfig (for NXP's ls1043a)

This patch fixes issue with defining the DM_SPI_FLASH in the
configs/include/<board.h> instead of enabling this option in Kconfig.

The problem is that CONFIG_IS_ENABLED(DM_SPI_FLASH) shows false as there
is no DM_SPI_FLASH=y in .config (but the define is set in u-boot.cfg).

As a result conversion of DM_SPI_FLASH to using CONFIG_IS_ENABLED() is not
working properly.

Signed-off-by: Lukasz Majewski <lukma@denx.de>